English: This animation depicts the time evolution of five-year averages of annual precipitation as simulated in the GFDL CM2.1 model for the years 1900 to 2100. These animations were produced using the output of the CM2.1 "H2" historical experiment for the 1901-2000 time period and the CM2.1 SRES A1B emissions scenario run for the 2001-2100 period. Results are shown as color changes on a world map. Future changes are measured according to how they compare to 20th century precipitation. Regions may be wetter or drier compared to the 20th century (further information: "Details on the simulation").

Warming of the global climate is expected to be accompanied by a reduction in rainfall in the subtropics and an increase in precipitation in subpolar latitudes and some equatorial regions. This pattern can be described in broad terms as the wet getting wetter and the dry getting drier, since subtropical land regions are mostly semi-arid today, while most subpolar regions currently have an excess of precipitation over evaporation. Though clearly a feature of a warming global climate, this characterization of the changing precipitation pattern cannot be applied to every locale, but should instead be thought of as a large-scale tendency that can be modified by local conditions in some cases. Drying is projected to be strongest near the poleward margins of the subtropics (for example, South Africa, southern Australia, the Mediterranean, and the south-western U.S.), a pattern that can be described as a poleward expansion of these semi-arid zones.

This large-scale pattern of change is a robust feature present in nearly all of the simulations conducted by the world's climate modeling groups for the 4th Assessment of the Intergovernmental Panel on Climate Change (IPCC), including those conducted at NOAA’s Geophysical Fluid Dynamics Laboratory (GFDL). It is also evident in observed 20th century precipitation trends.

Details on the simulation

The projected change in precipitation is in response to increasing atmospheric concentrations of greenhouse gases and aerosols based on a "middle of the road" estimate of future emissions. Future emissions are based on the "A1B" emissions scenario, taken from the Special Report on Emissions Scenarios.

Rather than mapping rainfall amounts in units of inches or millimeters, changes at each model grid point are assigned to one of five different categories (indicated by the five colors in the color bar at the bottom of the plot).

The categories were calculated as follows. After computing five year running averages of annual precipitation for each GFDL CM2.1 model grid point, the 1901-2000 period was analyzed. For each model grid point the maximum and minimum values for the 20th century were noted. Also, at each grid point, the individual 20th century five year rainfall values were ranked in order from driest to wettest, and then divided into three groups. Rainfall amounts falling in the driest third are designated below nomal (light tan), the wettest third are categorized as above normal (pale blue-green), and the middle third as near normal (off white). By definition these three middle colors will be the only ones would appear in maps the modeled 1901 to 2000 period.

After 2000, when a five year precipitation average is less than any of the one hundred five year running averages simulated for the 20th century, that point and time is classified as drier than the 20th century (saddle brown). Similarly, when a five year rainfall value is greater than any simulated for that grid point during 1901-2000 period, it is labelled wetter than the 20th century (darker blue-green).

References

• NOAA, February 2007: GFDL Climate Modeling Research Highlights: Will the wet get wetter and the dry drier? (PDF), vol. 1, no. 5, revision 10/15/2008, 4:47:16 PM. The National Oceanic and Atmospheric Administration (NOAA) Geophysical Fluid Dynamics Laboratory (GFDL) - Princeton, NJ, USA
• NOAA – see citation for the video